The present invention relates to a process for substantially reducing the level of organic and chlorinated organic contaminants, including color contaminants, in waste water effluent streams, by subjecting the effluent streams to a combination of ultraviolet light and hydrogen peroxide, optionally in the presence of catalysts. The invention is particularly adapted to treatment of effluent from a pulp mill but is also suitable in other applications, such as sewage treatment and in eliminating contaminants from ground water. The invention reduces the contaminant load to conventional waste water treatment facilities and, in the case of pulp mills, permits an increase in mill capacity without costly additional investment for expanding waste water treatment. In addition, reducing effluent contamination allows substantial recycling of treated effluent streams back to the mill.
Pulping and bleaching processes require large volumes of chemicals to produce bleached pulp suitable for producing paper. Because of this, both pulping and bleaching processes generate contaminated effluent requiring extensive treatment prior to discharge to the environment.
In the Kraft pulping process, currently the most prevalent process in the industry, sodium sulfide and sodium hydroxide (caustic soda) digest wood chips to form unbleached pulp. Effluent streams are created both in the preparation of the wood chips and in the recovery of spent liquor from the digesting process.
In the bleaching process, chlorine-based compounds are used extensively, with effluent streams being formed from various washing stages in the process. While chlorine-based bleaching compounds are very effective in pulp bleaching, they have the added effect of contaminating the effluent streams with chlorinated organics detrimental to the environment.
This occurs when the chlorine-based compounds react with lignin and other organic substances during bleaching. The result is the formation of so-called absorbable organic halogens (AOX) which accumulate in bleach plant effluent streams. Certain AOX have known carcinogenic effects in humans, and their level in the effluent streams is severely restricted.
Some organic contaminants also result in coloration of the effluent streams which has heretofore precluded substantial recycle of the effluent to the pulp mill , even after subsequent treatment.
AOX and other bleaching by-products, as well as any unreacted bleaching compounds, are removed from the bleached pulp by washing with water at various processing stages. The resulting effluent streams are then treated prior to discharge. Generally, these effluent streams are subjected to a waste water treatment process, and then discharged directly to the environment in large volume. Prior to discharge, the effluent streams are rigorously monitored for their biological oxygen demand (BOD) and chemical oxygen demand (COD), as well as for total organic carbon (TOC), effluent color, and toxicity, including AOX levels.
The primary means of reducing BOD, COD, TOC, color, AOX and toxicity in pulp mill effluent is through biological treatment. This normally requires large volume, aerated treatment lagoons with relatively long residence times to enable environmentally safe treated effluent discharge. As mills continue to expand pulp production capacities, their existing biological treatment steps are often unable to handle the corresponding increase in waste water influent and are unable to meet lower discharge levels.
For this reason, some mills have begun to employ chemical oxidants to enhance the contaminant removal capability of their existing waste water treatment lagoons. Chemical oxidants such as hydrogen peroxide, potassium permanganate, and ozone have been considered for this purpose. Chemical oxidants can be added directly to waste water influent, for example, to reduce its toxicity toward the resident microbial 10 population, or to reduce the contaminant burden prior to biological treatment. Alternatively, the oxidants can be added to effluent from biological treatment as a means of polishing the effluent, further reducing contaminants prior to discharge.
The use of these various chemical oxidants, while helpful in effluent treatment, are limited in their oxidation effect. Hence, chemical oxidants alone are insufficient for allowing anything but modest increases in pulp production capacity and/or for decreasing contaminant concentration in effluent discharge.
Another method for reducing the level of discharged contaminants in pulp mill effluent would be to recycle all or part of the effluent streams after treatment. Heretofore, this has not been considered a viable option, since no method has been available to reduce color and other contaminants to a low enough level. The chemical oxidants noted above are incapable of reducing contaminants to the required low levels. Also, the art did not think it possible that other chemical oxidants were capable of reducing contaminant concentration any further.
This conclusion stems from the nature of the unique contaminants in pulp mill effluent, making them very resistant to most chemical oxidants. Recycled effluent must have very low levels of color and other contaminants, otherwise the recycling would result in the production of off-specification pulp due to transfer of the color and other contaminants to the pulp, for example, during washing stages. Also, recycling the contaminant burden to the pulping and bleaching processes without increasing the amount of fresh water make-up, would cause an increase in the concentration of contaminants carried through the processes.
It has been known to combine the use of hydrogen peroxide with ultraviolet radiation in the treatment of sewage from naval vessels, as disclosed in U.S. Pat. No. 4,012,321, which patent is hereby incorporated by reference in its entirety. This patent teaches that irradiation of an aqueous waste stream (i.e., sewage) in which hydrogen peroxide has been added, facilitates the breakdown of the hydrogen peroxide to hydroxyl radicals (.cndot.OH).
U.S. Pat. No. 4,012,321 is particularly directed to the removal of acetic acid from sewage formed from human excrement. There is no indication in this patent that combined UV/H.sub.2 O.sub.2 treatment would be useful in the treatment of the contaminants in pulp mill effluent.
Other documents pertaining to the combined use of UV/H.sub.2 O.sub.2 include:
"The Oxidation Pre-Treatment of Organic Matter in Natural Waters", Tittle et al, Ion Exchange Technology, 1984, pp. 74-84; "Photoinduced Oxidation of Humic Acid in Water Using Hydrogen Peroxide and UV Irradiation", Morayama et al, Water Pollut. Dept. Nat'l. Res. Inst. Pollut. Resorv., Japan, Kogai 1985 20(i), 9-15; JP 62176595; DE 3501528; JP 60028883; JP 52035445; and JP 52016865. None of these documents, however, relates to the treatment of pulp mill effluent.
The use of various catalysts in the treatment of waste water is described in U.S. Pat. Nos. 5,043,080 and 5,266,214, both of which are incorporated herein by reference.
U.S. Pat. No. 5,043,080 relates to the treatment of liquid effluent or ground water with hydrogen peroxide and transition metal ions in the presence of UV radiation. There is no discussion of treatment of pulp mill effluent in this patent.
U.S. Pat. No. 5,266,214 relates to treatment of contaminated water with hydrogen peroxide and ferric oxalate in the presence of light, e.g., UV light. Again, this patent contains no teaching of the treatment of pulp mill effluent.